In the recent drive for higher integration and operating speeds in LSI devices, it is desired to miniaturize the pattern rule. The exposure process and the resist composition are largely altered to meet such a demand. Particularly when resist patterns with a feature size of 0.2 μm or less are formed by lithography, KrF and ArF excimer laser radiation, electron beam (EB) or the like is used as the energy source for exposure, and chemically amplified resist compositions having a high sensitivity to such high-energy radiation and affording a high resolution are used as the photoresist.
Resist compositions include positive ones in which exposed areas are dissolved away and negative ones in which exposed areas are left as a pattern. A suitable composition is selected among them depending on the desired resist pattern. In general, the chemically amplified negative resist composition comprises a polymer which is normally soluble in an aqueous alkaline developer, an acid generator which is decomposed to generate an acid upon exposure to light, and a crosslinker which causes the polymer to crosslink in the presence of the acid serving as a catalyst, thus rendering the polymer insoluble in the developer (sometimes, the crosslinker is incorporated in the polymer). Typically a basic compound is added for controlling the diffusion of the acid generated upon light exposure.
A number of negative resist compositions of the type comprising a polymer which is soluble in an aqueous alkaline developer and includes phenolic units as the alkali-soluble units were developed, especially as adapted for exposure to KrF excimer laser light. These compositions have not been used in the ArF excimer laser lithography because the phenolic units are not transmissive to exposure light having a wavelength of 150 to 220 nm. Recently, these compositions are recognized attractive again as the negative resist composition for the EB and EUV lithography capable of forming finer size patterns. Exemplary compositions are described in Patent Documents 1 to 3.
In the course of development of resist compositions as mentioned above, the resist compositions are required to exhibit not only a high resolution which is the fundamental function of a resist film, but also high etch resistance. This is because the resist film must be thinned as the pattern feature size is reduced. One known means for achieving such high etch resistance is by introducing a polycyclic compound containing aromatic ring and non-aromatic ring wherein the non-aromatic ring has a carbon-carbon double bond conjugated to the aromatic ring, like indene or acenaphthylene, into a hydroxystyrene-based polymer as an auxiliary component. Examples are described in Patent Documents 1 to 3. Units having phenolic hydroxyl group are disclosed in Patent Documents 4 and 5.
On the other hand, the chemically amplified resist composition typically contains a compound which is decomposed to generate an acid upon exposure to high-energy radiation, known as acid generator, the acid serving as the catalyst for changing the solubility of the polymer in developer. A number of acid generators have been developed. In the immersion lithography, one effective technique of preventing the acid generator from being leached out into the immersion medium is by incorporating the acid generator into a polymer as one of recurring units. While this technique was applied to the positive resist composition, it was found effective for improving line edge roughness (LER) (see Patent Documents 6 to 8).